I. Field of the Invention
The present invention is directed generally to the field of hydraulic systems operated by intermittently or continuously operated fluid pumps and, more particularly, to vehicle mounted hydraulic systems utilizing gear pumps which are rotated by the vehicle motor. The present invention focuses on a flow recirculation control system which allows essentially the pump demand to be met at all times. This prevents overheating and shortened pump life due to starvation/cavitation problems.
II. Discussion of the Related Art
Vehicle mounted hydraulic systems are typically powered by gear pumps. These hydraulic pumps may operate anytime the truck motor is running, i.e., continuously, or be driven from a power take-off (PTO) so that the pump can be turned off or mechanically disconnected when not in use. Such systems at a given time are typically characterized as being in one of three defined modes of operation. These include an ON mode in which the pump is running and one or more hydraulic devices are being operated, and an OFF mode in which the pump may or may not be running but no hydraulic devices are being operated. The third mode is characterized as an OVERSPEED mode in which the pump is turning at a speed at which, under normal conditions, it will deliver too much flow to the system with respect to the then current need.
Prior systems typically employ what is known as a "dry valve" system in which a valve located on the suction side of the pump--when the pump is in the OFF or OVERSPEED mode--meters a small amount of hydraulic fluid which is allowed to flow through the pump as a minimum supply when the system is running. Unfortunately, the minimum amount often is but a small fraction of the actual pump demand, so that the pump is forced to operate in a starved condition. This type of arrangement is typical of prior systems and, while operable, is undesirable because operation under starved condition may result in higher than normal pump operating temperatures and undue wear on the pump. For example, if the truck or other vehicle carrying a hydraulic system having a continuously operated pump is driving down the road at high speed and the hydraulic system is in the OFF mode, the pump could still be turning fast enough to ask for perhaps 60-70 gallons of oil per minute. The dry valve system, however, meters only an average of, for example, two gallons of oil per minute into the pump producing starvation cavitation which increases the noise, heat generated and wear on the pump parts.
An illustration of such a prior hydraulic system for one type of refuse truck is illustrated by the schematic hydraulic diagram of FIG. 1. The system includes a fluid reservoir 10 equipped with a vented reservoir filler cap 12 and return line filter 14. The reservoir 10 is connected to a reservoir outlet/pump inlet suction line 16. A dry valve is depicted at 18 connected to the inlet 20 of a gear pump 22. The outlet 23 of the gear pump 22 is connected by line 24 to the inlet section 26 of a first main spool valve which includes a truck tailgate operating section 28, a refuse ejector operating spool valve section 30 and an outlet section 32. The tailgate operating section 28 is connected to a tailgate operating cylinder 34 and the ejector section is connected with a telescoping ejector operating cylinder as at 36. An additional hydraulic pressure line 38 supplies high pressure hydraulic fluid to a packer/slide spool valve inlet section at 40 which connects with spool valve sections 42, 44, 46 and 48 which operate additional respective cylinders as illustrated at 50, 52 and 54 to provide other tipping, sliding, packing and, if desired, winch functions. A main low pressure hydraulic return line is shown at 56.
It is clear from the schematic diagram of FIG. 1 that all of the hydraulic fluid flowing from the pump outlet through high pressure line 24 must flow through the spool valves prior to returning to the reservoir 10. Demand may be adequate for pump output when one or more of the cylinders is being operated. During periods of no demand, when none of the hydraulic cylinders is being operated under this condition, the permissible flow through the pump is controlled by dry valve 18. The pump operates with an undersupply of hydraulic fluid and thereby experiences cavitation which causes the pump to be noisy, run at a much higher than desired temperature and generally shortens pump life. If low flow problems could be alleviated, the maintenance with regard to such system could be greatly reduced.